LT8302MPS8E 查看數據表（PDF） - Linear Technology

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LT8302MPS8E

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Linear Technology 

LT8302

Applications Information

Design Example

Use the following design example as a guide to designing

applications for the LT8302. The design example involves

designing a 5V output with a 1.5A load current and an

input range from 8V to 32V.

VIN(MIN) = 8V, VIN(NOM) = 12V, VIN(MAX) = 32V,

VOUT = 5V, IOUT = 1.5A

Step 1: Select the transformer turns ratio.

NPS

<

65V

–

VIN(MAX) – VLEAKAGE

VOUT + VF

VLEAKAGE = Margin for transformer leakage spike = 15V

VF = Output diode forward voltage = ~0.3V

Example:

NPS

<

65V – 32V – 15V

5V + 0.3V

=

3.4

The choice of transformer turns ratio is critical in determin-

ing output current capability of the converter. Table 2 shows

the switch voltage stress and output current capability at

different transformer turns ratio.

Table 2. Switch Voltage Stress and Output Current Capability vs

Turns Ratio

VSW(MAX) at

NPS

VIN(MAX) (V)

1:1

37.3

IOUT(MAX) at

VIN(MIN) (A)

0.92

DUTY CYCLE (%)

14-40

2:1

42.6

1.31

25-57

3:1

47.9

1.53

33-67

Clearly, only NPS = 3 can meet the 1.5A output current

requirement, so NPS = 3 is chosen as the turns ratio in

this example.

Step 2: Determine the primary inductance.

Primary inductance for the transformer must be set above

a minimum value to satisfy the minimum switch-off and

switch-on time requirements:

( ) LPRI

≥

tOFF(MIN)

• NPS • VOUT

ISW(MIN)

+

VF

LPRI

≥

tON(MIN) • VIN(MAX)

ISW(MIN)

tOFF(MIN) = 350ns

tON(MIN) = 160ns

ISW(MIN) = 0.87A

Example:

LPRI

≥

350ns

•

3 • (5V

0.87A

+

0.3V

)

=

6.4µH

LPRI

≥

160ns • 32V

0.87A

=

5.9µH

Most transformers specify primary inductance with a toler-

ance of ±20%. With other component tolerance considered,

choose a transformer with its primary inductance 40% to

60% larger than the minimum values calculated above.

LPRI = 9µH is then chosen in this example.

Once the primary inductance has been determined, the

maximum load switching frequency can be calculated as:

( ) fSW

=

tON

1

+ tOFF

=

LPRI • ISW

VIN

1

+

LPRI • ISW

NPS • VOUT +

VF

ISW

=

VOUT

η•

• IOUT •

VIN • D

2

8302fa

18

For more information www.linear.com/LT8302

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